1 Answer
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The code that are trying to include shows tags under each post and needs to be included somewhere between {block:Posts} and {/block:Posts}. In addition, your posts must have tags if you want tags to be shown (that's why it's called HasTags). Tags are to be specified in the dedicated field when you're creating or editing a post.

If you are talking about a tag cloud, then Tumblr doesn't have a built-in one. However, you can integrate a third-party one. The most popular one is by Heather Rivers and can be found on her site.